Technical Field
The present disclosure pertains to the generation of ultraviolet (UV), visible (VIS), and near-infrared (NIR) light and, more particularly, to a light source system that generates stable optical power over time and temperature.
Description of the Related Art
Constant-power light sources are desired for industrial, medical, and military sensing and measurement applications where the light power ideally remains unchanged for all operating temperatures. An example of industrial sensing wherein accuracy depends on constant light power is the determination of water clarity by optical means. In this application, a water sample is illuminated with a light source and the intensity of light scattered from the illuminated sample is measured with a photodetector. Clear water will scatter relatively little light and turbid water will scatter relatively more of it. An accurate measurement is obtained when the change in scattered light intensity results only from changes in water clarity and not from the intensity of the light source. Because the optical and electrical properties of real light-emitting semiconductors and optical materials vary with temperature, an absolutely stable light source does not exist. Instead, conventional sources compromise performance to achieve acceptable peak wavelength, bandwidth, power, stability, electrical power consumption, size, and cost. When small size, electrical-power consumption, and cost are the primary concerns, semiconductor devices are the light emitters of choice because the semiconductor chips have volumes less than 0.008 mm3 and they can emit conical light beams that can be collimated with simple optics. Laser diodes (LDs) and vertical-cavity surface-emitting lasers (VCSELs) are appropriate when coherent, polarized light is required and resonant-cavity (RCLEDs), edge-emitting light-emitting diodes (EELEDs), and super luminescent diodes (SLDs) are appropriate incoherent light sources. Hereafter, the term laser shall mean an LD or a VCSEL or both and the term incoherent emitter shall mean one or more of the devices comprising RCLEDs, EELEDs, and SLDs. Among the diverse applications of incoherent LED light are pulse oximeters, water turbidity sensors, and optical gyroscopes. Semiconductor lasers provide light for computer pointing devices and telecommunications.
Semiconductor light sources used for communications and industrial applications are driven with commercially available laser and LED drivers that produce power that varies by as much as 15% over 50° C. Improved control can be achieved by placing a temperature sensor near the light emitter and using an electronic circuit to vary the drive current through a semiconductor emitter to maintain nearly constant power. Such a method reduces power fluctuation to less than 2.5% over 50° C. with an open control loop as disclosed in U.S. Pat. No. 4,841,157. An open loop controller relies on inputs other than the output light power itself and cannot respond to deterioration of a semiconductor emitter during its life. Light-source systems having open-loop controllers will therefore drift unpredictably as the light emitter power declines with age, and the host sensing device will require periodic calibration.
In another conventional system having a closed-loop controller, a signal representing a fraction of the emitted light is compared to a preset signal and the light-emitter drive current is continuously adjusted electronically to bring the power to a desired constant value. A closed-loop controller takes a sample of the output, in this case optical power, converts it to an electrical signal, and feeds it back to an input of the light-emitter driver in a feedback loop. It is said to be closed because signals influencing its operation originate within the light-source system, and external environmental factors have negligible effects on it. A schematic of a conventional light-source system with closed-loop electro-optical feedback used for telecommunications and sensing applications is shown in FIG. 1, where wide arrows represent light beams and thin arrows represent electrical signals. The first element is a semiconductor light emitter 100 that is driven with electrical current 102 supplied by a driver circuit 104. The emitted light 106 is collimating by a lens 108 into a beam of parallel light rays 110 having power PL that impinges upon beamsplitter 112, which reflects a sample 114 of the light beam with power PR onto monitor photodetector 116 and transmits an output light beam 118 having power PO to an output device for the intended purpose. The light sample 114 generates a photocurrent 120 that flows through a sense resistor RS 122, creating sample signal VS 124. The signal 124 is connected to one input 126 of driver circuit 104 and a reference signal VREF 128 is connected to the other input 130. The driver circuit 104 supplies the light emitter 100 with current 102 so as to maintain the signals VS and VREF equal and produce constant light power PO. This conventional system compensates for the decline in light power of a semiconductor emitter with temperature and age.
In order to achieve complete stabilization over an operating temperature range, the light emitter 100, beamsplitter 112, monitor photodetector 116, and driver circuit 104 must be maintained at a constant temperature by a thermoelectric cooler (TEC) or by an equivalent means. A beamsplitter is an optical device, such as a glass plate or wedge, that intercepts a light beam and directs a sample of it to a detector where it is converted to an electrical signal for control purposes. A thermoelectric cooler (TEC) is used to stabilize the monitor-photodetector responsivity, the reflectance and transmittance of the optical coating, and the output optical power of a light source. The use of a TEC, however, substantially reduces the wall-plug efficiency of a light source.
This approach is common in nearly all light sources in conventional sensing applications and results in light-power fluctuations less than 5% over 50° C. while compensating for aging effects and alleviating the need for calibration. Examples of such designs are found in U.S. Pat. Nos. 5,209,112; 5,796,481; 6,222,202; and 6,483,862.
In U.S. Pat. No. 7,767,947, Downing describes a light source having a Fresnel plate beamsplitter and a closed-loop controller for stabilizing the power of a VCSEL to vary less than 2.5% over 50° C. The disclosed method compensates for VCSEL aging. In U.S. Pat. No. 8,502,452, Downing and Babic further describe a light source system, hereafter called the optical method, wherein a beam of polarization-locked, laser light impinges on an interference coating deposited on a glass wedge that reflects a sample of the light to a monitor photodetector and transmits a portion to an output device. Photocurrent from the monitor photodetector is input to a closed-loop controller that continuously adjusts the laser drive current such that its power is maintained substantially constant. The interference coating has transverse-electric (TE) and transverse-magnetic (TM) reflectance spectra that have opposite signs and thereby enables the polarization angle of the laser beam to be adjusted at the time of manufacture to minimize the variation of output light power to less than ⅛% over 50° C. A TEC is not required for light sources built by the optical method and as much as a watt of electrical power is conserved thereby substantially improving wall-plug efficiency.
A first manufacturing challenge of the optical method is that the coating reflectance can change as much as 1.8% (18,000 ppm) for a one-degree change of angle of incidence. This sensitivity demands an alignment tolerance of better than ±0.05° to achieve the claimed control accuracy. A second challenge is the spectral tolerance of coating reflectance, which can vary by several percentage points within a batch of beamsplitter coatings. This tolerance substantially reduces production yields and necessitates costly laser-alignment equipment.
Applications including, but not limited to, battery-powered water- and air-turbidity sensing, fog and visibility monitoring, and blood-gas analysis would significantly benefit from miniature devices about the size of a marble or a JEDEC TO-5 package (roughly 1 cm3) that can be powered by two AAA alkaline batteries for at least 50 hours. These requirements preclude the use of thermoelectric coolers (TECs) to stabilize wavelength and power because TECs consume as much as a watt of electrical power. In summary, a state of the art light source with peak wavelength variations less than 20 nm over 50° C. and light source power that can be stabilized to 25 ppm per ° C. exists but only at a high manufacturing cost that substantially limits the market. In sensors and measurement systems that can tolerate changes in wavelength less than about 20 nm in 50° C., the methods disclosed in the present implementation compensate for temperature-dependent control errors. The disclosed implementation achieves equivalent stability with coherent-polarized emitters (lasers) as well as incoherent emitters such as RCLEDs and SLDs but with significant manufacturing-cost reduction. This is achieved by virtually eliminating manufacturing tolerances for the coating, monitor photodiode, spectral characteristics of the semiconductor emitter, and alignment equipment as well as relaxing coating specifications when one is used. The disclosed features broaden the potential market for low-power ultra-stable optically based sensors and systems.